1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method, and a program for causing a computer to execute the wireless communication method, which allow a plurality of wireless communication apparatus on a wireless communication network to transmit data at data transmission intervals that are controlled according to an autonomous distributed control scheme so as to achieve high overall throughput of a communication system.
2. Description of the Related Art
In recent years, an improvement in the transmission rate of a wireless LAN system and a reduction in cost have been achieved, and, as a result, the wireless LAN system has become very popular in a wide variety of applications, and many new applications have been proposed. For example, a personal area network (PAN) has been proposed to realize a mall-sized wireless network that allows a plurality of electronic devices located around a user to communicate with each other. For the above purpose, there have been proposed various wireless communication systems using various frequency bands such as a 2.4 GHz band or a 5 GHz band that do not need a particular license for use.
Widely used standards for wireless communication networks are IEEE (the Institute of Electrical and Electronics Engineers) 802.11, HiperLAN/2, IEEE802.15.3, and Bluetooth. The IEEE 802.11 standard has extended standards such as IEEE802.11a, b, and g for various wireless communication modes and frequency bands.
For a wireless local area network, two communication modes are known: an infrastructure mode; and an ad hoc mode.
In the infrastructure communication mode, a wireless communication apparatus called an access point (AP) is installed in an area, and communication via a network is performed under the general control of the access point. More specifically, when a certain communicating station (a wireless communication apparatus) wants to transmit data, the communication stations requests the access point to reserve a band for use by the communication station to transmit the data without having a collision with data transmitted by other communication stations. This method is called a bandwidth reservation protocol. In this communication protocol, communication stations on the wireless communication network communicate synchronously with each other via the access point. In asynchronous communication between sending and receiving communication stations in such an infrastructure wireless communication system, because the communication is performed via the access point, it is difficult to use the transmission channel in an efficient manner.
On the other hand, in the ad hoc wireless communication mode, communication stations directly communicate with each other in an asynchronous manner. Because arbitrary communication stations are allowed to directly communicate with each other without using a particular access point, the ad hoc wireless communication mode is suitable in particular for a home network for communication among home appliances. In the ad hoc mode, even if a failure occurs in some communication station or electric power of some communication station is turned off, routing is automatically changed so as to adapt to the new situation, and thus the total network can still work. By hopping a packet from a communication station to another a plurality of times, it is possible to transmit data to a relatively distant location at a high transmission rate.
A typical media access control protocol for data transmission via a wireless network is the CSMA (Carrier Sense Multiple Access) protocol. In the CSMA protocol, multiple accessing based on carrier sensing is allowed. In wireless communication, it is difficult for a wireless communication apparatus to receive a signal transmitted from the communication apparatus itself. Therefore, in wireless communication, instead of employing CSMA/CD (Carrier Sense Multiple Access with Collision Detection) employed in Ethernet (registered trademark), CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is employed to avoid a collision. More specifically, in the CSMA/CA, a check is first performed as to whether no data is being transmitted from other wireless communication apparatus, and data is transmitted when no data being transmitted from other wireless communication apparatus is detected. The CSMA protocol is suitable for asynchronous communication for transferring files or transmitting e-mails.
In the IEEE802.11 standard, the CSMA/CA protocol is employed as a protocol for wireless channel access using the DCF (Distribution Coordination Function). When data is transmitted using the DCF, it is needed to check the usage status of a wireless communication channel (by performing media sensing) for a period equal to a predetermined frame interval (called a DIFS (DCF interface space)) plus a random time (called a back-off time) set for a wireless communication apparatus to confirm that no data is being transmitted from other wireless communication apparatus. The back-off time is given by the product of a random number generated within a predetermined range called a contention window (CW) (CWmin≦CW≦CWmax) and a unit time called a slot time.
Because the DIFS is set to a fixed value, if an equal back-off time is set for a plurality of wireless communication apparatus, there is a high probability that data is transmitted at the same time from the plurality of wireless communication apparatus and a data collision occurs on a wireless communication channel. In the CMSA/CA protocol, to avoid the above problem, the contention window value assigned for each wireless communication apparatus is doubled each time the wireless communication apparatus retransmits data to reduce the probability that the back-off time is set to a value equal to that for another wireless communication apparatus thereby reducing the probability that a data collision occurs. If a wireless communication apparatus succeeds in retransmitting data, an ACK signal indicating that the data has been successfully received is transmitted from a destination of data. In response to receiving the ACK signal, the contention window is initialized.
On the other hand, in the case of a wireless LAN system using a UWB (Ultra Wide Band) communication technique, it takes a long time to detect a preamble, and thus there is a high probability that a collision occurs. As a result, frequent retransmission of data occurs, which results in a reduction in the overall throughput of the system. To avoid the above problem, it is known to increase the contention window value depending on the number of wireless communication apparatus currently existing on a network thereby decreasing the probability that a collision occurs (see, for example, Japanese Unexamined Patent Application Publication No. 10-173663). In this technique, because the contention window value is increased with the number of the wireless communication apparatus, back-off times set to respective wireless communication apparatus are scattered over the increased contention window, and thus a reduction in the collision probability is achieved.